Spatial light modulator display devices include so-called "active matrix" devices, comprising an array of light modulating elements, or "light valves", each of which is controllable by a control signal (usually an electrical signal) to controllably reflect or transmit light in accordance with the control signal. A liquid crystal array is one example of an active matrix device; another example is the deformable mirror device (DMD) array developed by Texas Instruments and described in, for example, U.S. Pat. No. 4,615,595 or in "128.times.128 deformable mirror device" IEEE Trans. Electron Devices ED-30, 539 (1983), L J Hornbeck. One type of colour video projection display system using a DMD array is described in U.S. Pat. No. 4,680,579.
Generally speaking, a DMD array comprises a plurality of separately addressed electrically deflectable mirrors; if a light beam is directed on the array device, the light reflected in a given direction by the array will depend upon the angle of inclination of each mirror, so that a video image may be superimposed on a reflected beam of light by controlling the mirrors of the array in accordance with the pixel values of a frame of a video signal. To produce a grey scale of intensity, it is possible to control the angle of each mirror through a continuous range using an analog control signal but a greatly preferred alternative method, which gives better contrast, involves controlling the mirror deflection between two positions corresponding to substantially reflective and unreflective states, and controlling the length of time in which the mirror is in the reflective state. Although a pulse width modulation technique could be used, as described in "Deformable Mirror Device Spatial Light Modulators and Their Applicability to Optical Neural Networks", Collins et all, Applied Optics Vol. 28, No. 22, November 1989, page 4904, a preferred technique is disclosed in our United Kingdom Patent Application No. 9100188.3 filed on 4 January 1991 (agents ref. 3203201) incorporated herein by reference.
To produce a colour display, three separately illuminated active matrix devices may be provided, each controlled by a respective colour signal and illuminated by a respective colour light beam, the modulated colour light beams being recombined for display, for example as disclosed in U.S. Pat. No. 4,680,579 or in our UK patent applications 9101715.2 and 9101714.5 filed on 25 January 1991 (agents ref. 3203301 and 3203401), incorporated herein by reference.
One example of a spatial modulator device which is not an active matrix device is the "eidophor" system, in which an oil film is illuminated by a light beam and, simultaneously, scanned by an electron beam controlled by a video signal, which acts to control the reflective state of the oil film.
With projection systems in general, the overall light level in the projected image, as perceived by an observer in the viewing space (for example an auditorium) varies over the screen area. Relatively large scale intensity variations are caused by, for example, the reduction of light intensity towards the edge of the screen due to the physical geometry of the projection system (specifically the projection lens), which is known as "vignetting", and the variation of light intensity due to the variations of angles of incidence over the screen area. U.S. Pat. No. 4,210,928 shows a prior display system, using cathode ray tubes rather than light modulators to correct this type of intensity variation by providing a light blocking plate within the optical path of the projected light beam prior to the screen. However, even with cathode ray tubes this solution is crude, and of limited usefulness since it involves forming and/or mechanically aligning a high precision optical component for each combination of projection system and screen, which is expensive in applications such as audiotorium or cinema projection systems.
In active matrix displays such as DMD displays, where very small mechanical components are electrically deflected, there can be large variations in efficiency of deflection of pixel elements compared to a CRT due to mechanical factors.
Another problem is that the variation of the overall light level or intensity of the image on the screen varies differently when viewed from different positions in the viewing space. This is particularly so when a high gain viewing screen (in other words, one which has a substantially directional reflection characteristic, such as a "PEARLITE" screen) is employed; in general, most of the light is reflected normal to the screen with a sharp decrease in the intensity of reflection when the screen is viewed from one side. A correction which is calculated or derived to improve the viewing from one position within the viewing area may thus actually make matters worse at another point.
Another problem is that intensity variation on a very small scale can occur due to the variations in response of particular picture elements of the active matrix display device within the projector; this problem is particularly marked in presently available DMD devices since the mechanical and electrical responses of individual mirrors are sensitive to manufacturing variations and to environmental factors such as atmospheric moisture. Although the eye generally does not respond to a single isolated pixel intensity variation of this type, numbers of variations across the device give rise to the appearance of spatially static noise known variously as the "grain", "dirty window", "silk stocking" or "mosaic" effects.
EP0184901 proposes to correct a cathode ray rude (not for projection display applications) by initially taking a photograph of the response of the tube to a frame showing the peak amplitude for each colour using a video camera, and then digitising the camera output to provide a stored frame correction signal for each colour. In use, the stored frame correction signal is read out in synchronism with a video signal to be displayed, converted to an analog signal, and subtracted from the video signal. However, since a simultaneous picture of the cathode ray tube is taken, it is necessary that the camera be positioned directly in front of the cathode ray tube.